Not applicable.
The invention relates to a method for measuring and regulating curl in a paper or board web, in which method the curl of the paper or board web is regulated.
The invention also relates to a paper or board machine line which comprises sections for the manufacture of a paper or board web, which paper or board machine line comprises a means or means for regulating curl in the web.
It is known from experience that if paper is dried one-sidedly, the result is a tendency of curling of the sheet. When paper is dried, for example, by means of normal groups with single-wire draw from the side of its bottom face and if such asymmetric drying is extended over the entire length of the dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. The thus dried paper usually curls and becomes concave, when viewed from above.
Reducing the tendency of curling of paper, in particular of fine paper, has become more and more important along with new printing and copying methods, in which methods the paper is heated suddenly, as a rule from one side, so that any internal strains that may have remained in the paper because of different reasons are discharged in the form of its curling. The tendency of paper to curl is affected both by the anisotropies in the different properties of the structure of the paper, such as fibre orientation, filler distribution, and density, and by anisotropies produced in the paper during drying, both in the direction of the plane of the paper and in the z-direction.
As known in the prior art, the tendency of curling of paper is already affected in connection with the forming of the web, in particular at the sheet formation stage by means of the selection of the difference in speed between the slice jet and the wire, and by means of the choice of the former and its mode of running, as well as by means of other running parameters. As known in the prior art, for example, in the case of copying paper, by means of unequal sidedness of drying in the after-dryer, a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying might be optimized. In the case of copying paper, the reactivity of curling, i.e. the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper, which is produced in connection with the forming of the web in the wet end.
One factor that has a substantial effect on the curling of paper is then one-sided drying of the paper. In drying sections provided with the traditional twin-wire draw, the web can be dried at the same rate from both sides, and the uniformity of drying can be affected, and the tendency of paper to curl can be prevented, by regulating the steam pressures in the upper and lower cylinders. By means of the concept of a dryer section with single-wire draw, said drying at the same rate and said possibility of regulation cannot be accomplished. Attempts are made to prevent unequalsided drying, for example, by means of inverted drying groups, moistening and steam treatment of the web. For example, in connection with calendering, it is possible to employ steam spraying applied to the paper, the effect of which is based on relaxing the strains in the paper fibres by the effect of heat and moisture.
With respect to the prior art related to the invention in connection with regulation of curl and the methods used in it, reference may be made to FI patent 91900, FI patent applications 971301 and 972080 as well as to U.S. Pat. Nos. 5,269,074; 5,542,193; 5,557,860; and 5,592,571. These publications describe various arrangements and devices which are employed for regulation of paper curl. The principles described in said publications can naturally be also used for control of the curl profile in the cross machine direction. The publications also describe dryer sections known in the prior art in which control of curl is essential in order to achieve a good end product.
One problem in the prior art arrangements is that a method that is suitable for on-line measurement of curl is not known, which method has turned out to be an almost compelling need in modern paper manufacture. If curl could be measured on-line, it could also be regulated as quickly as possible as desired.
Thus, an object of the invention is to provide a method for measuring curl in a paper web, which method is suitable for use in connection with the manufacture of paper, i.e. on-line.
An object of the invention is also to provide a method which is suitable for use at several different locations in connection with a paper or board machine line.
A further object of the invention is to provide a paper or board machine line in which the curl of a web is measured on-line.
With a view to achieving the objectives stated above as well as those coming out later, the method according to the invention is mainly characterized in that, in the method, in connection with the manufacture of a paper or board web, the paper or board web is slit or cut, that the rising/lowering of the edges of a slit/cut gap formed at the slitting/cutting point or the width of the gap is measured by means of a measurement device, and that the curl of the paper or board web is regulated based on the result of measurement.
The paper or board machine line according to the invention is in turn mainly characterized in that the paper or board machine line comprises means for slitting or cutting a paper or board web and for measuring the rising/lowering of the edges of a slit/cut gap formed at the slitting/cutting point or the width of the slit/cut gap in connection with the manufacture of the web in order to regulate the curl of the web based on the result of measurement.
In accordance with the invention, the paper web is slit in its running direction over a suitable length and curl is determined based on the rising/lowering of the slit/cut edges or on the width of the slit/cut gap. The slit/cut edges rise/lower because of the tendency of paper to curl such that the result of measurement of the rising/lowering/width of the slit/cut gap is proportional to the intensity of curl. The necessary correlation may be determined based on laboratory measurements known in themselves. On the basis of on-line measurement, the factors affecting the curling of the paper web or one/some of them can be regulated in order to provide desired curl.
Suitable quick measurement, for example, laser measurement is used in the method according to the invention. The measurement device used is placed above or underneath a web at a location in a suitable way, so that the rising/lowering of the slitting point or of the slit/cut edges or the width of the slit/cut gap can be quickly measured. The measurement device may also be accomplished such that there is situated a light source/detector measurement device which is effective across the slit/cut gap, which device is quick and precise enough to determine the rising/lowering of the slit/cut edges or the width of the gap. Another measurement device known in itself, which can be advantageously utilized in the invention, is formed of a camera and image processing associated therewith.
In accordance with the invention, the curl of the paper web is regulated based on the result obtained from the measurement, for example, by regulating the steam pressure between upper/lower drying cylinders in dryer groups with twin-wire draw in a dryer section, by using a web moistening device or by regulating a device drying the web predominantly from one side. The methods and devices disclosed in the above-mentioned FI patent 91900, FI applications 971301 and 972080 as well as U.S. Pat. Nos. 5,269,074; 5,542,193; 5,557,860; and 5,592,571 can be applied in the invention.
in accordance with the invention, the measurement device is placed, in accordance with an advantageous embodiment example, on a paper machine at a location where the web is slit in any case, for example, in connection with a slitter-winder or the cutting of an edge strip. The invention naturally also encompasses arrangements in which devices are arranged at a desired measurement point, for example before a reel or a winder, for slitting the web at a desired point over a desired length. A slitter-winder and the point of cutting an edge strip are advantageous locations for placement because in that case there is no need for separate slitting/cutting devices and no extra paper broke is produced.
In accordance with the invention, the web may be slit or cut, for example, by a diagonal cutter, a separate slitter or cutter, a knife, or by using water jet cutting. After that, the slit/cut edges are examined by the measurement device which measures the rising/lowering of the edge or the width of the gap. After that, the regulation system is supplied with the measured value and data of the running conditions of the machine as well as with information, for example, based on laboratory results, on the relationship between the rising/lowering of the edges or the width of the gap and the actual curl. Based on the curl determined at the preceding stage, a desired curl regulation device or paper machine part/section is regulated in order to regulate the curl as desired, determining set values for curl regulation.
In accordance with the invention, it is also possible to establish a curl profile by slitting or cutting at several points or by passing the web almost diagonally with respect to the running direction of the web, whereby the curl profile may be measured over the entire width of the web and it may be regulated as desired by using devices/taking action that enable the web to be profiled in the width direction for the purpose of curl regulation.
When the method according to the invention is applied somewhere else but in connection with slitting-winding or cutting of an edge strip, the amount of broke produced is very small because the time required by the slitter/cutter and measurement devices advantageously used is, for example, about one second, in which connection 30 m of paper is manufactured at a speed of 1800 m/min.
The slitting of the web takes place, for example, by means of a blade or an equivalent slitter or cutter element or by means of a slitter-winder device or a device for cutting an edge strip, in which connection, when a blade or an equivalent cutter or slitter element is used, the slitting cutter device is struck quickly through the web, after which the edges of the slit web turn either upwards or downwards depending on the effect of curl. After that, the rising/lowering of the edge or the width of the slit/cut gap is measured by means of an optical measurement device, for example, utilizing a laser, in which connection the time taken by measurement is very short. The measurement device is placed either above or underneath the web or such as to measure across the web in the lateral direction, depending on the location in the paper or board machine line. Moreover, for instance, diagonal cutting devices in use today may be employed for slitting/cutting, in which connection there is no need for a separate slitter or cutter device.
The measurement location in the paper or board machine line may be, for example, after a dryer section, after a calender before a reel, between calenders or on a reeling or winding machine in connection with cutting of an edge strip.
When it is desired to measure the curl profile in the cross-machine direction from time to time, the cutting blade and the measurement device may be arranged to travel together across the web, for example, from the tending side to the driving side, thereby establishing the entire curl profile in the cross direction of the web.
When the invention is applied to a slitter-winder, the measurement device, for example, a laser measuring head can be arranged to be movable, so that it can be displaced from one slitting point to another, thereby enabling a rougher transverse profile of curl to be found out, depending on the number of the rolls to be slit. It is, of course, also possible to use several stationary measurement devices, for example, laser measurement devices.